Processes for producing olefins by the dehydrogenation of saturated hydrocarbons are well known. A typical dehydrogenation process mixes the feed hydrocarbons with hydrogen and heats the resulting admixture by indirect heat exchange with the effluent from the dehydrogenation zone. Following heating, the feed mixture passes through a heater to further increase the temperature of the feed components before it enters the dehydrogenation zone where it is contacted with the dehydrogenation catalyst. The catalyst zone may be operated with a fixed bed, a fluidized bed, or a movable bed of catalyst particles. After heat exchange with the feed, the dehydrogenation zone effluent passes to product separation facilities. The product separation facilities will typically produce a gas stream, made up primarily of hydrogen, a first product stream comprising the desired olefin products, and a second potential product stream comprising light hydrocarbons. The light hydrocarbon stream typically has fewer carbon atoms per molecule than the desired olefin product. Light hydrocarbons are generally removed from the product stream in order to reduce flow volume, operating pressures, and undesirable side reactions or coking in downstream process units that receive the olefin product. A portion of the hydrogen stream is typically recycled to the dehydrogenation zone to provide hydrogen for the combined feed stream. The product stream usually contains unconverted dehydrogenatable feed hydrocarbons in addition to the product olefin. These unconverted hydrocarbons may be withdrawn in the separation facilities for recycle to the dehydrogenation zone or passed together with the product olefins to hydrocarbon conversion processes that use the product olefins.
Dehydrogenated hydrocarbons are used in etherification processes for making high octane compounds which are used as blending components in lead-free gasoline. These etherification processes will usually produce ethers by combination of an isoolefin with a monohydroxy alcohol. The etherification process can also be used as a means to produce pure isoolefins by separation and subsequent cracking of the product ether. For instance, pure isobutylene can be obtained for the manufacture of polyisobutylenes and tert-butyl-phenol by cracking methyl tertiary butyl ether (MTBE). The production of MTBE has emerged as a predominant etherification process which uses C.sub.4 isoolefins as the feedstock. A detailed description of processes, including catalyst, processing conditions, and product recovery, for the production of MTBE from isobutylene and methanol are provided in U.S. Pat. Nos. 2,720,547 and 4,219,678 and in an article at page 35 of the June 25, 1979 edition of Chemical and Engineering News. The preferred process is described in a paper presented at The American Institute of Chemical Engineers, 85th National Meeting on June 4-8, 1978, by F. Obenaus et al. Another etherification process of current interest is the production of tertiary amyl ether (TAME) by reacting C.sub.5 isoolefins with methanol.
Due to the limited availability of olefins for olefin conversion processes, such as etherification, it has become common practice to combine a dehydrogenation zone and an olefin conversion process. General representations of flow schemes where a dehydrogenation zone effluent passes to an etherification zone are shown in U.S. Pat. Nos. 4,118,425 4,447,653 and 4,465,870. U.S. Pat. Nos. 4,447,653 and 4,465,870 issued to Vora and Herskovits, respectively, represent the usual arrangement of dehydrogenation and etherification combinations that employ fractionation for deethanization and depropanization between dehydrogenation and etherification facilities. More complete representations of a flow arrangement where the dehydrogenation zone effluent passes to an etherification zone are given in U.S. Pat. No. 4,329,516 and at page 91 of the October, 1980 edition of Hydrocarbon Processing. The latter two references also depict the typical gas compression and separation steps that are used to remove hydrogen and light ends from the dehydrogenation zone effluent before it passes to the etherification zone. A typical effluent from an etherification zone includes an ether product, unreacted alcohol, and unreacted hydrocarbons which include light ends. These effluent components enter separation facilities that yield the ether product, alcohol for recycling to the etherification zone, hydrocarbons for further processing including dehydrogenation, and light ends which can be further separated into fuel and chemical feedstocks.
It is a broad object of this invention to improve the arrangement and operation of a combination process for dehydrogenating hydrocarbons and reacting the dehydrogenated hydrocarbons.
A more specific object of this invention is to simplify the separation facilities in a combined process for the dehydrogenation of dehydrogenatable hydrocarbons and the etherification of the dehydrogenated hydrocarbons.